Photon-counting computed tomography (CT) apparatuses are known that are CT apparatuses employing photon-counting detectors measuring x-rays as photons. An indirect conversion photon-counting detector includes a combination of a scintillator and a photo-detector. The scintillator converts radiation passing through a subject into scintillation light (photons) in proportion to energy of the radiation. Photons emitted by the scintillator enter the photo-detector and a pulse signal is obtained that has a pulse height in proportion to the energy of the radiation. The energy of the radiation entering the photon-counting detector can be measured by analyzing the waveform of the pulse signal output from the photon-counting detector.
The photon-counting detector is superior in detecting photons when an amount of radiation is small. An increase in the amount of radiation causes the photon-counting detector to output a single signal instead of a plurality of pulse signals because the number of radiation photons is counted small due to pulse-pileup of the photon-counting detector. This miss counting, thus, increases the difference between the number of radiation photons entering the photon-counting detector and the number of photons actually detected by the photon-counting detector.
As a radiation detection device included in the CT apparatus, a charge-integrating (energy-integrating) detector is also used. An indirect conversion charge-integrating detector outputs the total amount of the received photons as an electrical signal. Charges generated in the detector are accumulated for a certain time in a capacitor connected to the charge-integrating detector.
The charge-integrating detector is superior in detecting photons when an amount of radiation is large. Unlike the photon-counting detector, the charge-integrating detector accurately detects photons even if the amount of radiation increases. Instead, a signal/noise (S/N) ratio of the electrical signal deteriorates when the amount of radiation is small.
A radiation detection device is desired that can complement shortcomings in respective performances of the photon-counting detector and the charge-integrating detector.
A method is available that performs switching in such a manner that the photon-counting detector functions when an amount of radiation entering the radiation detection device is small while the charge-integrating detector functions when the amount of radiation entering the radiation detection device is large. For switching the photon-counting detector and the charge-integrating detector, it is necessary to switch voltages applied to photo diodes included in the respective photon-counting detector and charge-integrating detector. The radiation detection device fails to detect photons during the switching of the photon-counting detector and the charge-integrating detector. As a result, detection efficiency of photons is reduced.
The embodiment aims to increase the detection efficiency of photons without needing to switch the photon-counting detector and the charge-integrating detector.